The thick film dielectric structure provides for superior resistance to dielectric breakdown as well as a reduced operating voltage as compared to thin film electroluminescent (TFEL) displays, as exemplified by U.S. Pat. No. 5,432,015. The thick film dielectric structure also enhances the amount of charge that can be injected in to the phosphor film to provide greater luminosity than can be realized from TFEL displays. Full colour thick dielectric electroluminescent displays are described for example in the Applicant's co-pending International Patent Application WO 2004/036961. These displays employ high luminance blue phosphor materials to directly illuminate blue sub-pixels and colour conversion materials to down-convert the blue light to red or green light for the red and green sub-pixels. A preferred blue phosphor material for use in thick film dielectric electroluminescent displays is europium activated barium thioaluminate.
Thick film electroluminescent displays fully meet the luminosity and colour spectrum capability of cathode ray tube (CRT) based displays. However, the operating stability still falls short of that provided by CRTs.
Magnesium oxide layers have been used in organic electroluminescent displays to protect organic electroluminescent displays from ambient moisture and magnesium oxide layers have been used in thin film structures for displays such as those described in U.S. Pat. Nos. 4,429,303, 4,547,702, 4,849,674, 5,319,282, 5,466,990, 5,190,333, 6,087,766, 6,023,258, 6,111,353, 6,207,302, 6,147,456, 6,414,442, and U.S. Patent Applications 2003/0073042 and 2004/0159903, but have not been reported for use in inorganic electroluminescent displays to provide chemical isolation between the inorganic phosphor film and adjacent dielectric layers, particularly thick film dielectric layers that are chemically complex and have a tendency to react with adjacent phosphor layers during display fabrication and subsequently during display operation.
Alumina layers have been used to provide a measure of chemical isolation between thick dielectric layers and phosphor layer, however, their effectiveness is reduced when the phosphor is deposited by sputtering rather than by evaporation. The phosphor sputtering process may subject the underlying thin dielectric layer with bombardment by energetic ions that may cause deterioration of the thin dielectric layer. Magnesium oxide has been used to coat phosphor powders used in colour plasma displays to protect the phosphor materials from the harsh environment of the UV-emitting pixel plasma.
There remains a need to provide further improvements to thick film dielectric electroluminescent displays that will further improve the luminance of phosphors provided therein and also extend their operating life with minimal degradation.